1. Field of the Invention
The present invention relates to a mini-lamp or night light with a fluorescent tube which is capable of automatic turn-on/turn-off operation, e.g., turn-on when the ambient light falls below a certain level and turn-off when the ambient light rises above a certain level.
2.Description of the Prior Art and Related Information
Fluorescent lamps of all types are very popular for use in the home or office because of their high operating efficiency as compared to incandescent lamps. Indeed, fluorescent lamps emit light at several times the efficiency of a typical incandescent lamp. Furthermore, fluorescent lamps do not generate as much heat as a typical incandescent bulb, thereby conserving radiant energy in that respect.
A typical fluorescent lamp is constructed from a glass tube that contains two electrodes at opposite ends, a coating of powdered phosphor covering the interior of the tube, and small amounts of mercury. The electrodes when energized provide a large potential across the tube which frees electrons to initiate an arc. The radiant energy from the arc contains shortwave ultraviolet energy that is converted into light by the phosphor coating. In this process, the fluorescent effect is caused by the mercury when it is vaporized in the arc.
Due to their operating requirements, fluorescent lamps are not usually used when automatic on/off operation is required or desired. For this reason, night lights, for which it is desired to have automatic on/off operation, typically utilize incandescent lamps with a photocell, with the photocell operating as an on/off switch. That is, when ambient light is above a predetermined level, the photocell causes power to be removed from the incandescent lamp, and when ambient light is below a predetermined level, the photocell causes power to be applied to the incandescent lamp. However, photocells are not suitable for use with fluorescent lamps in this manner because of the higher voltages and currents involved. The prior art method of providing an automatic on/off feature for a fluorescent lamps utilizes a bimetal thermocouple device in series with a cadmium sulfide (CdS) photoelectric device to switch the lamp on and off as shown in FIG. 1. The bimetal thermocouple device functions as a relay. A conventional relay could be used, with suitable modifications. However, the resulting device would be cost prohibitive.
The bimetal thermocouple device works as follows with reference to FIG. 1. As ambient light decreases to a predetermined level, the resistance of a CdS photoelectric device increases causing a decrease in current flow. This causes resistor R to cool down which in turn causes the bimetal thermocouple to contact one terminal of the fluorescent lamp thereby completing the circuit and switching fluorescent lamp F on. In a similar manner, as the ambient light rises above a predetermined level, the resistance of the CdS photoelectric device decreases. The resulting current increase flowing through resistor R, causes it to heat up which causes the bimetal thermocouple to lose contact with the terminal of the fluorescent lamp. This switches the lamp off. In FIG. 1, starter ST and inductor L are conventional components used in fluorescent lamp circuits in a manner well known to persons skilled in the art. Although such configuration works, the resistor R-bimetal thermocouple combination takes several minutes for the contact to the fluorescent lamp to be made or broken. Since consumers expect night lights to turn on as soon as a room light is switched off, the delay associated with a bimetal thermocouple is generally not acceptable. While a relay could provide an acceptable response time, relays are usually too expensive for devices such as night lights.